Serial serum specimens collected 15 years ago from untreated patients with acute schistosomiasis mansoni have been previously studied for the evolution of their IgG and IgE antibody responses to different schistosome antigens. Because of the recently recognized importance of IgG4 antibodies both for diagnostic specificity and for blocking allergic reactions to the parasite, this unique collection of sera has been restudied to define the kinetics of a developing specific IgG4 antibody response. Interestingly, it appears that while antibodies of 1gG1, IgG2 and IgG3 subclasses develop within weeks of acute infection, specific IgG4 antibodies (to both egg and adult schistosome antigen) do not develop for 6-9 months after infection. This piece of information is of particular interest, since there are almost no other instances where the natural development of an IgG4 response in humans has been recorded.